The present invention relates to GPS measurement systems, and more particularly with a measurement system and a method for tracking an object using a number of multi-frequency antennas.
The Global Positioning System (GPS) is a satellite based navigation system having a constellation of 24 Earth orbiting satellites. These satellites are approximately uniformly dispersed around six circular orbits having four satellites each. Theoretically, four or more GPS satellites are visible from most points on the Earth's surface.
Each GPS satellite presently transmits at two frequencies: L1 (1575.42 MHz) and L2 (1227.60 MHz). There exists provision (for the future) for a third frequency L5 (1176.45 MHz) as well. The L1 frequency has two different spread-spectrum codes modulated on it: a coarse acquisition (C/A) code and a Y code. The C/A code is an unclassified code intended for civilian navigation. It has a chipping rate of 1.023 MHz and a sequence length of 1023 chips. The Y code is a classified unknown code; people doing research in this area have found it to be the product of two codes: a precise (P) code and a W code. The P code is an unclassified code with a chipping rate of 10.23 MHz. The P code is long enough that it does not repeat during a week; it is reset at the beginning of the GPS week for each satellite. The P code is mixed with the classified W code to get an encrypted Y code. The W code has been empirically found to have a chipping rate of approximately 500 KHz. The Y code is modulated onto the L1 carrier in quadrature with the C/A code and with half the power of the C/A code. The Y code is also modulated onto the L2 carrier signal with half the power of L1 Y code. Both C/A and P codes are unique for each satellite.
GPS receivers are commonly used for a variety of applications involving tracking of the position of various objects. The object to be tracked is coupled to one or more GPS antennas that receive signals from the GPS satellites. Depending upon the level of accuracy and response time desired by a user, an appropriate method of obtaining position of an object using GPS can be adopted.
A commonly used method that yields the position information (within meters) is the pseudorange method. This method utilizes the C/A code and/or the P code modulated onto the carrier signals from the GPS satellites.
Use of a reference antenna that employs carrier phase measurements and has known coordinates can further enhance the accuracy of position determination of the antennas. Differential carrier phase GPS measurement is a technique which determines the position of a given antenna with respect to a reference antenna. The other antennas, known as roving antennas, are free to roam around. Measurements of the carrier phase at the reference antenna and the roving antennas are used to calculate the relative position of the antennas to centimeter level accuracy. Before the carrier phase measurements can be used for determining position accurately, the carrier cycle ambiguity or the number of complete carrier cycles between the antennas (reference antenna and roving antennas) must be determined.
Typically, the conventional approach for resolving carrier cycle ambiguities starts with a code-based differential GPS solution. Thereafter, the integer count for all the L1 satellite signals used in the position solution is determined. The integer solution is often ambiguous due to errors induced by receiver noise and multipath for both code and carrier based measurements. The integer solution is averaged over a period of time to converge on the exact solution. This process benefits from the intervening satellite motion. However, the process may take from a single measurement to several minutes worth of data to yield the correct integers depending on the number of satellites, the quality of the phase measurements and the desired level of confidence.
Dual frequency receivers that utilize both L1 and L2 frequency signals can determine carrier cycle ambiguities much faster than a single frequency receiver. A technique that uses both L1 and L2 phase measurements is faster than the one using just L1 carrier phase measurements. The phase of the L2 carrier is used to assist in resolving the carrier cycle ambiguity of the L1 signals.
The L1 carrier can be recovered by using a standard code correlation technique as the C/A code is known for each of the satellites. The L2 carrier signal is encrypted, thus only military GPS receivers that are aware of the encryption key can reconstruct the L2 signal with high accuracy. Civilian receivers can also reconstruct the L2 carrier signal using any of the known standard techniques, most of which derive the L2 carrier using the L1 carrier. However, the signal to noise ratio (SNR) of resulting L2 signal is lower than L2 signals reconstructed using military receivers.
Typically, GPS receivers employ dedicated RF sections for both the L1 and the L2 frequencies for every antenna to be tracked. The RF section down converts L1 and/or L2 RF signal and samples the signals for further processing. However, it may be prohibitively expensive to have a dedicated L1/L2 RF section for each antenna to be tracked. Moreover, in applications where it is desirable to track the position of multiple GPS antennas on a moving platform, the resulting system becomes very bulky. GPS receivers used for such applications usually employ multiplexing of both L1 as well as L2 signals to reduce the hardware cost. A patent that refers to such a GPS receiver is U.S. Pat. No. 6,154,170 titled ‘Enhanced Attitude Determination System Using Satellite Navigation Receiver With Antenna Multiplexing’, granted to Trimble Navigation Limited, Sunnyvale, Calif. Yet another patent that describes a system that multiplexes RF signals for multiple antennas is U.S. Pat. No. 5,917,448 titled ‘Attitude Determination System With Sequencing Antenna Inputs’, granted to Rockwell Science Center Inc. of Thousand Oaks, Calif.
Although, some of the abovementioned patents do refer to multiplexing for reducing the hardware cost, the SNR for L1/L2 measurements is low as the RF sections receive signals only for a fraction of the time. Hence, there exists a need for a system that derives L1/L2 signals with a high SNR and low hardware cost.